1. Field of the Invention
This invention relates generally to analog-to-digital converters and more particularly to a digital compensation type analog-to-digital converter which can calculate circulation type self calibration values with improved accuracy.
2. Description of the Related Art
The digitalization of audio and video appliances has increased the demand for high-speed and high-resolution analog-to-digital converters (A/D converters). In order to perform high resolution conversions, A/D converters have been implemented utilizing multistage converters. A multibit digital-to-analog converter (MDAC) is used to implement a multistage converter. However, the linearity characteristics of the entire converter are likely to deteriorate if any mismatch exists in the MDAC. Thus, self calibration techniques have been used to correct errors caused by the mismatch in the MDAC.
In a conventional self calibration technique, calibration values are generated through the same process as the normal conversion of the A/D converter. In such a process, if the amount of error in the MDAC is large, the generated calibration values correctly correspond to the actual values of the errors. However, if the error is small, correct calibration may not be effected. For example, if a positive error exists, the actual A/D-converted error value might be "0011111" in view of the characteristics of the A/D converter, even though this error value should be converted to "0100000" to be stored in memory. Consequently, the error value generated during the calibration value creating process is larger than the existing error value.
More specifically, there may be two elements of the A/D converter, each of which has an associated error. The A/D converter may determine that the error value for each of the elements is -1, even though each error value is actually close to "0" when the reference error is set to "0100000". If the two elements having respective errors are used separately in the A/D converter, the determined error value does not matter much since the incorrectly determined error value does not cause the A/D converter to exceed a predetermined nonlinearity limit. However, if the two elements are used simultaneously, the error value is determined to be "-2" as the sum of the two errors, rather than "-1" which is the true error. Thus, the predetermined nonlinearity limit is exceeded.
Such inaccurately determined error values do not matter much with respect to each individual calibration value. However, when they are summed together to perform self-calibration, the determined errors become greater than the existing errors, and as a result, the calibration process itself becomes useless.
Accordingly, a need remains for a technique for determining calibration values for an A/D converter which overcomes the problems mentioned above.